Many biological and chemical samples are stored at or below freezing temperatures. Generally speaking, a regular freezer operates from −5° C. to −20° C., an ultra-low temperature freezer operates from about −50° C. to −130° C. (preferably about −80° C.) and a cryogenic freezer operates from about −140° C. to −196° C. (the boiling point of liquid nitrogen). Most biological samples stored in ultra-low temperature or cryogenic systems are contained in sealed plastic laboratory tubes held in tube storage racks in arrays of, for example, 48, 96 or 384 tubes. In the art, it is known to robotically store tube storage racks on shelves or in removable storage cassettes within ultra-low temperature or cryogenic freezer compartments. Known storage cassettes typically have vertically aligned metal shelves with metal sidewalls. The top of the cassette usually has a handle for manual or robotic lifting of the cassette. A typical height of the prior art storage cassettes is about 3 feet in order to enable manual handling of the cassettes. Front and rear walls are typically open to allow access to the storage tube racks. Some cassettes have open shelves and some have closed shelves.
The present invention as mentioned is directed to storage cassettes for holding sample storage tube racks or plates within horizontal freezer compartments and for robotically transporting sample storage tube racks and plates into and from the freezer compartments. The primary objectives of the invention are to provide sample safety and thermal integrity while moving the storage cassettes from and into the freezer compartments. In order to accommodate a large number of samples, storage cassettes made in accordance with the invention can typically be about six feet long with, for example, twenty (20) or more shelves for holding tube storage racks and/or plates, although the cassettes can be made shorter (e.g. 3 feet) or even taller if less or more storage capacity per cassette is desired. One of the problems that must addressed when robotically transporting storage cassettes is that frost formation and thermal expansion and contraction can lead to difficulties when placing storage cassettes into storage racks in ultra-low temperature or cryogenic freezers. To address this problem, the invention provides a storage cassette that can reliably be inserted and removed from freezer compartments despite dimensional differences caused by thermal contraction and expansion, robotic misalignment, relatively minor frost formation or other difficulties that can be encountered. Reliability of insertion and removal is important because the time to remedy a jam can result in sample temperature rise which in turn can compromise sample integrity. The preferred embodiment of the invention includes not only features to facilitate reliable insertion and removal of storage cassettes from freezer compartments, but also features to reduce temperature rise in samples when the cassette is pulled from the freezer compartment.